Multifrequency EPR, ENDOR, ELDOR and Mossbauer spectroscopic and high pressure cryogenic technques are employed in combination with advanced numerical analytic methods to study the components and interactions of a variety of redox proteins, and in the process determine structural parameters of the active sites and elucidate protein mechanisms. During these studies, very strong emphasis is placed on spectral fitting techniques to computer simulate the output of the EPR and Mossbauer spectrometers in terms of physical models. The systems under study include: the cytochrome oxidases from beef-heart mitochondria and Thermus thermophilus, the hydrogenases from V. vulgaris and M. elsdenii, the cytochrome c3 from D. vulgaris, succinic dehydrogenase from T. thermophilus, several bacterial 2-, 4- and 8-iron ferredoxins, the High Potential Iron Protein from C. vinosum and alloxazine and isoalloxazine free radicals.